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The Prophet ﷺ said:
مَنْ حَفِظَ عَشْرَ آيَاتٍ مِنْ أَوَّلِ سُورَةِ الْكَهْفِ عُصِمَ مِنَ الدَّجَّالِ
Man hafiza 'ashra ayatin min awwali suratil-kahfi 'usima minad-Dajjal
Whoever memorises ten verses from the beginning of Sūrat al-Kahf will be protected from the Dajjāl.
Another narration says "from the end of al-Kahf". Also seek refuge in Allah from the trial of the Dajjāl after the final tashahhud of every prayer (see readings 55 and 56).
Source Ṣaḥīḥ Muslim, The Book of Prayer - Travellers — site no. 1775 Ṣaḥīḥ · Ṣaḥīḥ Muslim, The Book of Prayer - Travellers — site no. 1776 Ṣaḥīḥ · Sunan Abī Dāwūd 4323 (similar wording) Ṣaḥīḥ
Compiler cites Muslim 1/555; another narration has 'from the end of Sūrat al-Kahf' (1/556)